Phased arrays are used in naval ships, military aircraft, and ground-based locations to detect and track targets, friend and foe, and to guide missiles to enemy targets. They are also used in various non-military applications such as, for example, commercial air and terrestrial traffic detection and control, satellite and space probe communications, weather research and forecasting, radio broadcasting, and cellular communications.
A phased array comprises an array of antenna elements configured to radiate a plurality of electromagnetic waves of different phases. The antenna elements are spatially separated but proximate, thereby allowing the plurality of electromagnetic waves to interfere, constructively or destructively, depending on their phase relationships, to form a radiation pattern or “beam.” The direction of the beam (i.e., the “beam angle”) can be varied (i.e., “steered”), if necessary or desired, by simply changing the phase relationships among the various electromagnetic waves radiated by the antenna elements. Modern phased arrays are “electronically scanned” arrays (ESAs), meaning that solid-state devices are employed to control the phase relationships among the electromagnetic waves. Using solid-state devices, the beam can be steered very rapidly, both in elevation and azimuth, thus allowing a large portion of the sky or space to be scanned quickly without having to move or alter the physical orientation of the antenna array.
In general, there are two different types of ESAs—the passive electronically scanned array (PESA) and the active electronically scanned array (AESA). PESAs operate by: (1) dividing a high-power radio frequency (RF) signal generated by a single high-power microwave generator (such as produced by a klystron, magnetron, or traveling wave tube) into a plurality of high-power RF signals, (2) selectively delaying some of the divided high-power RF signals, and (3) finally, coupling the delayed and un-delayed high-power RF signals to a plurality of associated antenna elements. AESAs are more state-of-the-art, employing a high-power amplifier (HPA) in each transmit path to amplify a low-power RF source to higher power (rather than relying on a klystron or other similar large, high-power microwave generator to generate the high RF transmit power). AESAs also exploit the high-frequency capability and small form factor offered by modern integrated circuit technology, thereby allowing, for each antenna element, the HPA in the transmit path and front-end of the receive path to be assembled together in a single transmit-receive module (TRM).
FIG. 1 is a drawing showing how a plurality of TRMs 102 is configured in a conventional AESA 100. Each TRM 102 has an associated antenna element 104, which is used for both transmitting and receiving. During transmitting, a beamformer 106 divides a low-power RF transmit signal provided by a low-power RF source 108 into a plurality of low-power RF transmit signals for the transmit paths of the TRMs 102. The beamformer 106 also serves to set and control the individual amplitude and phase of each of the low-power RF transmit signals, in order to affect the direction of transmission (i.e., beam angle) of the final high-power RF beam that is ultimately radiated by the AESA 100. The amplitude and/or phase-adjusted RF transmit signals are converted to higher RF power by the HPAs 112 (which are configured for Class-A, B or AB operation). Finally, the high-power RF transmit signals are transduced into high-power electromagnetic waves by the antenna elements 104 and radiated into the air or space, where they then interfere to form the final high-power RF beam.
When receiving, the antenna elements 104 capture electromagnetic energy from impinging electromagnetic waves and convert the captured electromagnetic energy into low-power RF electrical signals. Because the electromagnetic energy captured by the antenna elements 104 is usually very weak, the received RF electrical signals must be amplified by LNAs 114 before they can be further processed downstream. The LNAs 114 are designed for low noise figure and are incapable of handling high input powers. Accordingly, to protect the LNAs 114 from being damaged and prevent receiver desensitization, the received RF signals are passed through limiters 116 before being directed into the inputs of the LNAs 114. Note that the amplitudes and/or phases of the received RF signals can also be adjusted by the beamformer 106 during receiving in order to adjust the receive array pattern.
One serious problem associated with the conventional TRM 102 is that the high-power RF transmit signal produced by its HPA 112 can be reflected by the TRM's antenna element 104 and back toward the output of the HPA 112, instead of being fully transduced and radiated into the air or space by the antenna element 104. The reflected RF signal is highly undesirable since it can distort the RF transmit signal being produced by the HPA 112 and cause its phase to deviate from its intended phase, either due to the reflected RF signal reflecting, once again, from the output port of the HPA 112 and combining with the intended RF transmit signal or by entering into the HPA 112 through the HPA's output port, which can give rise to intermodulation distortion. Another serious and related problem associated with the conventional TRM 102 is that when the TRM 102 is configured in an array, along with other TRMs, as in FIG. 1, RF signals transmitted from an antenna element 104 of one TRM 102 can be undesirably intercepted by antenna elements 104 of other TRMs 102 in the array. These “reverse” signals are also highly undesirable since they can be passed into the intercepting TRM's transmit path and also distort the RF transmit signal being produced by the HPA 112.
In an effort to prevent these problems from happening, circulators must be employed in the conventional TRM 102. In other words, as illustrated in FIG. 1, each TRM 102 of the AESA 100 must be equipped with its own circulator 118. A circulator 118 is a three-port device having a first port connected to the output of the HPA 112 (transmit path port) of its associated TRM 102, a second port connected to the TRM's antenna element 104 (antenna port), and a third port connected to the input of the receive path of the TRM 102 (receive path port). A two-port component with similar properties is an isolator, which transmits RF power from one port to a second, while blocking transmission in the opposite direction. The directional properties of the circulator 118 (and isolator) are asymmetric (i.e., are non-reciprocal). This asymmetry is exploited and relied on in the conventional TRM 102 to prevent reflected transmit signals and reverse signals received from other TRMs from being directed into the transmit path of the HPA 112. When a TRM 102 is transmitting, its circulator 118 provides a low-loss path for signals directed from the circulator's transmit path port to its antenna port, thereby allowing transmitting to occur, but isolates the transmit port from the antenna port in the reverse direction, for example, by attenuating signals (such as reflected signals or reverse signals) flowing in the reverse direction.
Although the circulator 118 in each TRM 102 can help to prevent reflected and reverse signals from entering the transmit path, the isolation it provides is only effective over a very narrow range of frequencies. This narrowband limitation of the circulator and/or isolator is highlighted in FIGS. 2A and 2B, which are scattering parameter measurements taken on a typical isolator. FIG. 2A shows the forward transfer coefficient (scattering parameter (S-parameter) s21) of the isolator swept over a 900 MHz to 1 GHz frequency range. FIG. 2B shows the reverse transfer coefficient (S-parameter s12) of the isolator swept over the same frequency range. As can be seen in FIG. 2A, the forward transfer coefficient s21 remains flat and near 0 dB (˜−0.27 dB) over the entire swept frequency range, indicating that for the forward direction, if a circulator with such characteristics was to serve as one of the circulators 118 in one of the TRMs 102 of the phased array system 100 in FIG. 1, it would be effective at delivering most of the power from port 1 (attached to transmit path port) to port 2 (attached to antenna port). However, FIG. 2B reveals that the reverse transfer coefficient s12 provides high isolation in the reverse direction only over a very narrow isolation bandwidth of ˜30 MHz.
One of the advantages of the AESA over the PESA is that because the TRMs 102 are independent, they can potentially be operated at different frequencies. In theory, this frequency division possibility would allow the AESA 100 to track more than one target at a time; allow it to produce a radiation pattern having a lower probability of intercept compared to if only a single frequency was used; and allow it to thwart jammers by employing a number or sequence of frequencies that is difficult or nearly impossible for a potential jammer to predict. Unfortunately, due to the limited reverse isolation capability offered by circulators, these various advantages of the AESA cannot be fully realized. It should be mentioned that this limited reverse isolation characteristic is not only a problem particular to radar applications, it is also a problem in circumstances where the AESA 100 is being used for data communications since distortion that results by not being able to isolate the output of the HPAs 112 from reflected and reverse signals can make it difficult, and in some cases even impossible, to satisfy signal accuracy requirements imposed by applicable communications standards.
In addition to its limited reverse isolation capability, circulators are large passive devices that consume power, occupy large areas of the printed circuit board (PCB) on which they and other components of the TRMs 102 are formed, and add to the overall weight and size of the AESA 100. As can be seen in FIG. 3, in a typical TRM 300 the circulator 304 and associated PCB waveguide traces occupy nearly a third of the area of the PCB 302. Moreover, because the HPA 112 of the TRM 102 is a linear amplifier, it is large in size and very energy inefficient. A large heatsink 306 is therefore required to conduct heat away from the HPA 112 and protect it from being damaged, and a larger power supply than desired is required to compensate for the HPA's low energy efficiency. The large circulator 304 and large heatsink 306 of each TRM 102 also have a substantial effect on the cost, size and weight of the AESA 100, when considered as a whole. An AESA will often include hundreds and sometimes thousands of TRMs, so the incremental cost, size and weight of each TRM must be multiplied by hundreds or thousands of times in determining the overall cost, size and weight of the AESA as a whole. Furthermore, with hundreds and possibly thousands of very inefficient linear HPAs involved, large, heavy and expensive cooling systems are necessary to displace the enormous amount of heat generated by the hundreds and possibly thousands of HPAs.
From the foregoing remarks it should be clear that while the need for circulators in conventional TRMs is necessary, their inclusion introduces a number of other very significant problems. It should also be clear that a primary reason why a circulator 118 is needed in the conventional TRM 102 is to isolate the HPA 112 and prevent its output from being exposed to reflected and reverse signals. Without the circulators 118 (or some other type of isolation devices) in the TRMs 102, the reflected and reverse signals would distort the RF transmit signals produced by the HPAs 112 and, depending on the level of distortion, render the AESA 100 dysfunctional and possibly even entirely inoperable. It would be desirable, therefore, to have an HPA for a TRM which when configured for operation in an AESA operates without the need for a circulator or other isolation device and produces an RF output that is resilient and substantially immune to the presence of reflected and reverse signals. It would also be desirable for such an HPA to be capable of providing this resiliency over a wide range of operating frequencies. The present invention is directed at an HPA that has these desired attributes.